1. Field of the Invention
The present invention relates generally to a system for heat treating coiled springs, such as steel automotive and truck suspension springs, brake springs, automotive power springs, and the like. The present invention also relates to a method and apparatus for resistance hardening coiled steel springs.
2. Discussion of the Related Art
Steel brake springs, suspension springs, and other automotive springs may be manufactured according to a "hot coil" process, an "annealed wire" process, or a "pre-tempered wire" process. In the hot coil process, straight steel bars are heated by natural gas or induction to a temperature in the range of from 1,600 to 1,900 degrees Fahrenheit (.degree. F.). The hot bars are then coiled into the desired shape, then quenched in oil, and then tempered. The hot coil process may be used to produce straight-sided springs; it has not been used effectively to produce variable body diameter springs.
In the annealed wire process, steel springs are first cold formed and then austenitized, quenched and tempered. The austenitizing step may be performed in either a batch furnace or a continuous furnace. The steel may be heated to a temperature in the range of from 1,500 to 1,620.degree. F. Unlike the hot coil process, the annealed wire process may be used to produce variable body diameter springs. The annealed wire process involves difficult material handling steps, however, and it may be subject to quality control problems. In particular, springs produced according to the annealed wire process may be subject to lot-to-lot inconsistency, decarburization, hardness non-uniformity, and distortion.
In the pre-tempered wire process, steel springs are cold coiled from pre-tempered wire. After coiling, the springs are stress relieved at a temperature in the range of from 700 to 800.degree. F. In the pre-tempered wire process, the steel material is hardened before it is coiled. The pre-tempered wire process is not economical and has other disadvantages.